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rfc:rfc7044

Internet Engineering Task Force (IETF) M. Barnes Request for Comments: 7044 Polycom Obsoletes: 4244 F. Audet Category: Standards Track Skype ISSN: 2070-1721 S. Schubert

                                                                   NTT
                                                         J. van Elburg
                                            Detecon International Gmbh
                                                           C. Holmberg
                                                              Ericsson
                                                         February 2014
     An Extension to the Session Initiation Protocol (SIP) for
                    Request History Information

Abstract

 This document defines a standard mechanism for capturing the history
 information associated with a Session Initiation Protocol (SIP)
 request.  This capability enables many enhanced services by providing
 the information as to how and why a SIP request arrives at a specific
 application or user.  This document defines an optional SIP header
 field, History-Info, for capturing the history information in
 requests.  The document also defines SIP header field parameters for
 the History-Info and Contact header fields to tag the method by which
 the target of a request is determined.  In addition, this
 specification defines a value for the Privacy header field that
 directs the anonymization of values in the History-Info header field.
 This document obsoletes RFC 4244.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7044.

Barnes, et al. Standards Track [Page 1] RFC 7044 History-Info February 2014

Copyright Notice

 Copyright (c) 2014 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.
 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Barnes, et al. Standards Track [Page 2] RFC 7044 History-Info February 2014

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
 2.  Conventions and Terminology . . . . . . . . . . . . . . . . .   4
 3.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   5
 4.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   6
 5.  History-Info Header Field Protocol Structure  . . . . . . . .   7
   5.1.  History-Info Header Field Example Scenario  . . . . . . .  10
 6.  User Agent Handling of the History-Info Header Field  . . . .  12
   6.1.  User Agent Client (UAC) Behavior  . . . . . . . . . . . .  12
   6.2.  User Agent Server (UAS) Behavior  . . . . . . . . . . . .  12
   6.3.  Back-to-Back User Agent (B2BUA) Behavior  . . . . . . . .  12
 7.  Proxy/Intermediary Handling of History-Info Header Fields . .  13
 8.  Redirect Server Handling of History-Info Header Fields  . . .  13
 9.  Handling of History-Info Header Fields in Requests and
     Responses . . . . . . . . . . . . . . . . . . . . . . . . . .  14
   9.1.  Receiving a Request . . . . . . . . . . . . . . . . . . .  14
   9.2.  Sending a Request with History-Info . . . . . . . . . . .  14
   9.3.  Receiving a Response with History-Info or Request
         Timeouts  . . . . . . . . . . . . . . . . . . . . . . . .  15
   9.4.  Sending History-Info in Responses . . . . . . . . . . . .  16
 10. Processing the History-Info Header Field  . . . . . . . . . .  16
   10.1.  Privacy in the History-Info Header Field . . . . . . . .  16
     10.1.1.  Indicating Privacy . . . . . . . . . . . . . . . . .  16
     10.1.2.  Applying Privacy . . . . . . . . . . . . . . . . . .  17
   10.2.  Reason in the History-Info Header Field  . . . . . . . .  18
   10.3.  Indexing in the History-Info Header Field  . . . . . . .  19
   10.4.  Mechanism for Target Determination in the History-Info
          Header Field . . . . . . . . . . . . . . . . . . . . . .  21
 11. Application Considerations  . . . . . . . . . . . . . . . . .  22
 12. Application-Specific Usage  . . . . . . . . . . . . . . . . .  24
   12.1.  PBX Voicemail  . . . . . . . . . . . . . . . . . . . . .  24
   12.2.  Consumer Voicemail . . . . . . . . . . . . . . . . . . .  25
 13. Security Considerations . . . . . . . . . . . . . . . . . . .  25
 14. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  26
   14.1.  Registration of New SIP History-Info Header Field  . . .  26
   14.2.  Registration of "history" for SIP Privacy Header Field .  27
   14.3.  Registration of Header Field Parameters  . . . . . . . .  27
 15. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  27
 16. Changes from RFC 4244 . . . . . . . . . . . . . . . . . . . .  28
   16.1.  Backwards Compatibility  . . . . . . . . . . . . . . . .  29
 17. References  . . . . . . . . . . . . . . . . . . . . . . . . .  31
   17.1.  Normative References . . . . . . . . . . . . . . . . . .  31
   17.2.  Informative References . . . . . . . . . . . . . . . . .  31
 Appendix A.  Request History Requirements . . . . . . . . . . . .  33
   A.1.  Security Requirements . . . . . . . . . . . . . . . . . .  34
   A.2.  Privacy Requirements  . . . . . . . . . . . . . . . . . .  35

Barnes, et al. Standards Track [Page 3] RFC 7044 History-Info February 2014

1. Introduction

 Many services that SIP is anticipated to support require the ability
 to determine why and how a SIP request arrived at a specific
 application.  Examples of such services include (but are not limited
 to) sessions initiated to call centers via "click to talk" SIP
 Uniform Resource Locators (URLs) on a web page, "call history/
 logging"-style services within intelligent "call management" software
 for SIP user agents (UAs), and calls to voicemail servers.  Although
 SIP implicitly provides the retarget capabilities that enable SIP
 requests to be routed to chosen applications, there is a need for a
 standard mechanism within SIP for communicating the retargeting
 history of the requests.  This request history information allows the
 receiving application to obtain information about how and why the SIP
 request arrived at the application/user.
 This document defines a SIP header field, History-Info, to provide a
 standard mechanism for capturing the request history information to
 enable a wide variety of services for networks and end-users.  SIP
 header field parameters are defined for the History-Info and Contact
 header fields to tag the method by which the target of a request is
 determined.  This specification also defines a value, "history", for
 the Privacy header field.  In addition, a SIP option tag, "histinfo",
 is defined.
 The History-Info header field provides a building block for
 development of SIP-based applications and services.  The requirements
 for the solution described in this specification are included in
 Appendix A.  Example scenarios using the History-Info header field
 are available in [CALLFLOWS].

2. Conventions and Terminology

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in [RFC2119].
 The term "retarget" is used in this specification to refer to the
 process of a SIP entity changing the Request-URI (Section 7.1 of
 [RFC3261]) in a request based on the rules for determining request
 targets as described in Section 16.5 of [RFC3261] and of the
 subsequent forwarding of that request as described in step 2 in
 Section 16.6 of [RFC3261].  This includes changing the Request-URI
 due to a location service lookup and redirect processing.  This also
 includes internal (to a proxy/SIP intermediary) changes of the URI
 prior to the forwarding of the request.

Barnes, et al. Standards Track [Page 4] RFC 7044 History-Info February 2014

 The terms "location service", "forward", "redirect", and "AOR"
 (address-of-record) are used consistently with the terminology in
 [RFC3261].
 The term "target user" is used in this specification as the human
 user associated with one or more particular AORs (in case the human
 user has multiple aliases).
 The references to "domain for which the SIP entity/proxy/intermediary
 is responsible" are consistent with and intended to convey the same
 context as the usage of that terminology in [RFC3261].  The
 applicability of History-Info to architectures or models outside the
 context of [RFC3261] is outside the scope of this specification.

3. Background

 SIP implicitly provides retargeting capabilities that enable SIP
 requests to be routed to specific applications as defined in
 [RFC3261].  The motivation for capturing the request history is that
 in the process of retargeting a request, old routing information can
 be forever lost.  This lost information may be important history that
 allows elements to which the request is retargeted to process the
 request in a locally defined, application-specific manner.  This
 document defines a mechanism for transporting the request history.
 Application-specific behavior is outside the scope of this
 specification.
 Current network applications for other protocols provide the ability
 for elements involved with the request to obtain additional
 information relating to how and why the request was routed to a
 particular destination.  The following are examples of such
 applications:
 1.  Web "referral" applications, whereby an application residing
     within a web server determines that a visitor to a website has
     arrived at the site via an "associate" site that will receive
     some "referral" commission for generating this traffic.
 2.  Email relaying whereby the recipient obtains a detailed "trace of
     the path" of the message from originator to receiver, including
     the time of each relay.
 3.  Traditional telephony services such as voicemail, call-center
     "automatic call distribution", and "follow me"-style services.
 Several of the aforementioned applications currently define
 application-specific mechanisms through which it is possible to
 obtain the necessary history information.

Barnes, et al. Standards Track [Page 5] RFC 7044 History-Info February 2014

 In addition, request history information could be used to enhance
 basic SIP functionality by providing the following:
 o  Some diagnostic information for debugging SIP requests.
 o  Capturing aliases and Globally Routable User Agent URIs (GRUUs)
    [RFC5627], which can be overwritten by a registrar or a "home
    proxy" (a proxy serving as the terminal point for routing an
    address-of-record) upon receipt of the initial request.
 o  Facilitating the use of limited use addresses (minted on demand)
    and sub-addressing.
 o  Preserving service-specific URIs that can be overwritten by a
    downstream proxy, such as those defined in [RFC3087], and control
    of network announcements and Interactive Voice Response (IVR) with
    a SIP URI [RFC4240].

4. Overview

 The fundamental functionality provided by the request history
 information is the ability to inform proxies and user agents (UAs)
 involved in processing a request about the history or progress of
 that request.  The solution is to capture the Request-URIs, as a
 request is retargeted, in a SIP header field: History-Info.  This
 allows for the capturing of the history of a request that would be
 lost with the normal SIP processing involved in the subsequent
 retargeting of the request.
 The History-Info header field is added to a request when a new
 request is created by a User Agent Client (UAC) or forwarded by a
 proxy, or when the target of a request is changed.  It is possible
 for the target of a request to be changed by the same proxy/SIP
 intermediary multiple times (referred to as 'internal retargeting').
 A SIP entity changing the target of a request in response to a
 redirect also propagates any History-Info header field from the
 initial request in the new request.  The ABNF and detailed
 description of the History-Info header field parameters, along with
 examples, are provided in Section 5.  Sections 6, 7, and 8 provide
 the detailed handling of the History-Info header field by SIP user
 agents, proxies, and redirect servers, respectively.
 This specification also defines three new SIP header field
 parameters, "rc", "mp", and "np", for the History-Info and Contact
 header fields to tag the method by which the target of a request is
 determined.  Further detail on the use of these header field
 parameters is provided in Section 5.

Barnes, et al. Standards Track [Page 6] RFC 7044 History-Info February 2014

 This specification also defines a priv-value for the Privacy header,
 "history"; it requires anonymization of all the History-Info header
 field entries in a request or to a specific History-Info header field
 value (hi-entry) as described below.  Further detail is provided in
 Section 10.1.
 In addition, a SIP option tag, "histinfo", is defined.  The use of
 this option tag is described in Section 6.1.

5. History-Info Header Field Protocol Structure

 The History-Info header field defined in this specification defines
 the usage in out-of-dialog requests or initial requests for a dialog
 (e.g., INVITE, REGISTER, MESSAGE, REFER and OPTIONS, PUBLISH and
 SUBSCRIBE, etc.) and any non-100 provisional or final responses to
 these requests.
 The following provides details for the information that is captured
 in the History-Info header field entries for each target used for
 forwarding a request.
 o  hi-targeted-to-uri: A mandatory parameter for capturing the
    Request-URI for the specific request as it is forwarded.
 o  hi-index: A mandatory parameter for History-Info reflecting the
    chronological order of the information, indexed to reflect the
    forking and retargeting of requests.  The format for this
    parameter is a sequence of nonnegative integers, separated by dots
    to indicate the number of forward hops and retargets.  This
    results in a tree representation of the history of the request,
    with the lowest-level index reflecting a leaf.  By adding the new
    entries in chronological order (i.e., following existing entries
    per the details in Section 10.3), including the index and sending
    the messages using a secure transport, the ordering of the
    History-Info header fields in the request is assured.  In
    addition, applications may extract a variety of metrics (total
    number of retargets, total number of retargets from a specific
    branch, etc.) based upon the index values.
 o  hi-target-param: An optional parameter reflecting the mechanism by
    which the Request-URI captured in the hi-targeted-to-uri in the
    History-Info header field value (hi-entry) was determined.  This
    parameter is either an "rc", "mp", or "np" header field parameter,
    which is interpreted as follows:

Barnes, et al. Standards Track [Page 7] RFC 7044 History-Info February 2014

       "rc": The hi-targeted-to-URI represents a change in
       Request-URI, while the target user remains the same.  This
       occurs, for example, when the user has multiple AORs as an
       alias.  The "rc" header field parameter contains the value of
       the hi-index in the hi-entry with an hi-targeted-to-uri that
       reflects the Request-URI that was retargeted.
       "mp": The hi-targeted-to-URI represents a user other than the
       target user associated with the Request-URI in the incoming
       request that was retargeted.  This occurs when a request is
       statically or dynamically retargeted to another user
       represented by an AOR unassociated with the AOR of the original
       target user.  The "mp" header field parameter contains the
       value of the hi-index in the hi-entry with an
       hi-targeted-to-uri that reflects the Request-URI that was
       retargeted, thus identifying the "mapped from" target.
       "np": The hi-targeted-to-URI represents that there was no
       change in the Request-URI.  This would apply, for example, when
       a proxy merely forwards a request to a next-hop proxy and loose
       routing is used.  The "np" header field parameter contains the
       value of the hi-index in the hi-entry with an
       hi-targeted-to-uri that reflects the Request-URI that was
       copied unchanged into the request represented by this hi-entry.
       That value will usually be the hi-index of the parent hi-entry
       of this hi-entry.
 o  Extension (hi-extension): A parameter to allow for future optional
    extensions.  As per [RFC3261], any implementation not
    understanding an extension MUST ignore it.
 The ABNF syntax [RFC5234] for the History-Info header field and
 header field parameters is as follows:
 History-Info = "History-Info" HCOLON hi-entry *(COMMA hi-entry)
 hi-entry = hi-targeted-to-uri *(SEMI hi-param)
 hi-targeted-to-uri = name-addr
 hi-param = hi-index / hi-target-param / hi-extension
 hi-index = "index" EQUAL index-val
 index-val =  number *("." number)
 number =  [ %x31-39 *DIGIT ] DIGIT

Barnes, et al. Standards Track [Page 8] RFC 7044 History-Info February 2014

 hi-target-param = rc-param / mp-param / np-param
 rc-param = "rc" EQUAL index-val
 mp-param = "mp" EQUAL index-val
 np-param = "np" EQUAL index-val
 hi-extension = generic-param
 The ABNF definitions for "generic-param", "name-addr", "HCOLON",
 "COMMA", "SEMI", and "EQUAL" are from [RFC3261].
 This document also extends the "contact-params" for the Contact
 header field as defined in [RFC3261] with the "rc", "mp", and "np"
 header field parameters defined above.
 In addition to the parameters defined by the ABNF, an hi-entry may
 also include a Reason header field and/or a Privacy header field,
 which are both included in the "headers" component of the
 hi-targeted-to-uri as described below:
 o  Reason: An optional parameter for History-Info, reflected in the
    History-Info header field by including the Reason header field
    [RFC3326] included in the hi-targeted-to-uri.  A reason is
    included in the hi-targeted-to-uri of an hi-entry to reflect
    information received in a response to the request sent to that
    URI.
 o  Privacy: An optional parameter for History-Info, reflected in the
    History-Info header field values by including the Privacy header
    [RFC3323] with a priv-value of "history", as defined in this
    document, included in the hi-targeted-to-uri or by adding the
    Privacy header field with a priv-value of "history" to the
    request.  The latter case indicates that the History-Info entries
    for all History-Info entries whose hi-targeted-to-uri has the same
    domain as the domain for which the SIP entity processing the
    message is responsible MUST be anonymized prior to forwarding,
    whereas the use of the Privacy header field included in the hi
    -targeted-to-uri means that a specific hi-entry MUST be
    anonymized.
 Note that since both the Reason and Privacy parameters are included
 in the hi-targeted-to-uri, these fields will not be available in the
 case that the hi-targeted-to-uri is a Tel-URI [RFC3966].

Barnes, et al. Standards Track [Page 9] RFC 7044 History-Info February 2014

 The following provides examples of the format for the History-Info
 header field.  Note that the backslash, CRLF, and whitespace between
 the lines in the examples below are inserted for readability purposes
 only.  Note, however, that History-Info can be broken into multiple
 lines due to the SWS (sep whitespace) that is part of HCOLON, COMMA,
 and SEMI, and there can be multiple History-Info header fields due to
 the rule of Section 7.3 of [RFC3261].  Additional detailed examples
 are available in [CALLFLOWS].
 History-Info: <sip:UserA@ims.example.com>;index=1;foo=bar
 History-Info: <sip:UserA@ims.example.com?Reason=SIP%3B\
               cause%3D302>;index=1.1,\
               <sip:UserB@example.com?Privacy=history&Reason=SIP%3B\
               cause%3D486>;index=1.2;mp=1.1,\
               <sip:45432@192.168.0.3>;index=1.3;rc=1.2

5.1. History-Info Header Field Example Scenario

 The following is an illustrative example of usage of History-Info.
 In this example, Alice (sip:alice@atlanta.example.com) calls Bob
 (sip:bob@biloxi.example.com).  Alice's proxy in her home domain
 (sip:atlanta.example.com) forwards the request to Bob's proxy
 (sip:biloxi.example.com).  When the request arrives at
 sip:biloxi.example.com, it does a location service lookup for
 bob@biloxi.example.com and changes the target of the request to Bob's
 Contact URIs that were provided as part of normal SIP registration.
 In this example, Bob is simultaneously contacted on a PC client and
 on a phone, and Bob answers on the PC client.
 One important thing illustrated by this call flow is that without
 History-Info, Bob would "lose" the original target information or the
 initial Request-URI, including any parameters in the Request-URI.
 Bob can recover that information by locating the last hi-entry with
 an "rc" header field parameter.  This "rc" header field parameter
 contains the index of the hi-entry containing the lost target
 information, i.e., the sip:bob@biloxi.example.com hi-entry with
 index=1.1.  Note that in the 200 response to Alice, an hi-entry is
 not included for the fork to sip:bob@192.0.2.7 (index 1.1.1) since
 biloxi.example.com had not received a response from that fork at the
 time it sent the 200 OK that ultimately reached Alice.
 Additional detailed examples are available in [CALLFLOWS].
    Note: This example uses loose routing procedures.

Barnes, et al. Standards Track [Page 10] RFC 7044 History-Info February 2014

 Alice   atlanta.example.com  biloxi.example.com   Bob@pc  Bob@phone
 |                |                |                |          |
 |   INVITE sip:bob@biloxi.example.com;p=x          |          |
 |--------------->|                |                |          |
 | Supported: histinfo             |                |          |
 | History-Info: <sip:bob@biloxi.example.com;p=x>;index=1      |
 |                |                |                |          |
 |                |   INVITE sip:bob@biloxi.example.com;p=x    |
 |                |--------------->|                |          |
 | History-Info: <sip:bob@biloxi.example.com;p=x>;index=1      |
 | History-Info: <sip:bob@biloxi.example.com;p=x>;np=1;index=1.1
 |                |                |                |          |
 |                |                |   INVITE sip:bob@192.0.2.3|
 |                |                |--------------->|          |
 | History-Info: <sip:bob@biloxi.example.com;p=x>;index=1
 | History-Info: <sip:bob@biloxi.example.com;p=x>;np=1;index=1.1
 | History-Info: <sip:bob@192.0.2.3>;index=1.1.1;rc=1.1
 |                |                |                |          |
 |                |                |   INVITE sip:bob@192.0.2.7|
 |                |                |-------------------------->|
 | History-Info: <sip:bob@biloxi.example.com;p=x>;index=1
 | History-Info: <sip:bob@biloxi.example.com;p=x>;np=1;index=1.1
 | History-Info: <sip:bob@192.0.2.7>;index=1.1.2;rc=1.1
 |                |                |     200        |          |
 |                |                |<---------------|          |
 | History-Info: <sip:bob@biloxi.example.com;p=x>;index=1
 | History-Info: <sip:bob@biloxi.example.com;p=x>;np=1;index=1.1
 | History-Info: <sip:bob@192.0.2.3>;index=1.1.1;rc=1.1
 |                |                |                |          |
 |                |     200        |                |          |
 |                |<---------------|                |          |
 | History-Info: <sip:bob@biloxi.example.com;p=x>;index=1
 | History-Info: <sip:bob@biloxi.example.com;p=x>;np=1;index=1.1
 | History-Info: <sip:bob@192.0.2.3>;index=1.1.1;rc=1.1
 |                |                |                |          |
 |                |                | Proxy Cancels INVITE      |
 |                |                |<=========================>|
 |     200        |                |                |          |
 |<---------------|                |                |          |
 | History-Info: <sip:bob@biloxi.example.com;p=x>;index=1
 | History-Info: <sip:bob@biloxi.example.com;p=x>;np=1;index=1.1
 | History-Info: <sip:bob@192.0.2.3>;index=1.1.1;rc=1.1
 |     ACK        |                |                |          |
 |--------------->|    ACK         |                |          |
 |                |--------------->|     ACK        |          |
 |                |                |--------------->|          |
                        Figure 1: Basic Call

Barnes, et al. Standards Track [Page 11] RFC 7044 History-Info February 2014

6. User Agent Handling of the History-Info Header Field

 This section describes the processing specific to UAs -- User Agent
 Clients (UACs), User Agent Servers (UASs), and Back-to-Back User
 Agents (B2BUAs) -- for the History-Info header.

6.1. User Agent Client (UAC) Behavior

 The UAC MUST include the "histinfo" option tag in the Supported
 header field in any out-of-dialog requests or initial requests for a
 dialog for which the UAC would like the History-Info header field in
 the response.  When issuing a request, the UAC MUST follow the
 procedures in Section 9.2.  In the case of an initial request, except
 where the UAC is part of a B2BUA, there is no cache of hi-entries
 with which to populate the History-Info header field, and the
 hi-index is set to 1 per Section 10.3.  When receiving a response,
 the UAC MUST follow the procedures in Section 9.3.
 If the UAC generates further forks of the initial request (either due
 to acting on a 3xx response or internally directed forking to
 multiple destinations), the successive requests will add hi-entries
 with hi-indexes of 2, 3, etc.

6.2. User Agent Server (UAS) Behavior

 When receiving a request, a UAS MUST follow the procedures defined in
 Section 9.2.  When sending a response other than a 3xx response, a
 UAS MUST follows the procedures defined in Section 9.4.  When sending
 a 3xx response, the UAS MUST follow the procedures defined for a
 redirect server per Section 8.  An application at the UAS can make
 use of the cached hi-entries as described in Section 11.

6.3. Back-to-Back User Agent (B2BUA) Behavior

 A B2BUA MAY follow the behavior of a SIP intermediary, per Section 7,
 as an alternative to following the behavior of a UAS per Section 6.2
 or a UAC per Section 6.1.  In behaving as an intermediary, a B2BUA
 carries forward hi-entries received in requests at the UAS to
 requests being forwarded by the UAC, as well as carrying forward
 hi-entries in responses received at the UAC to the responses
 forwarded by the UAS, subject to privacy considerations per
 Section 10.1.

Barnes, et al. Standards Track [Page 12] RFC 7044 History-Info February 2014

7. Proxy/Intermediary Handling of History-Info Header Fields

 This section describes the procedures for proxies and other SIP
 intermediaries for the handling of the History-Info header fields for
 each of the following scenarios:
 Receiving a Request:  An intermediary MUST follow the procedures in
    Section 9.1 for the handling of hi-entries in incoming SIP
    requests.
 Sending a Request:  For each outgoing request relating to a target in
    the target set, the intermediary MUST follow the procedures of
    Section 9.2.
 Receiving a Response or Timeout:  An intermediary MUST follow the
    procedures of Section 9.3 when a SIP response is received or a
    request times out.
 Sending a Response:  An intermediary MUST follow the procedures of
    Section 9.4 for the handling of the hi-entries when sending a SIP
    response.
 In some cases, an intermediary may retarget a request more than once
 before forwarding, i.e., a request is retargeted to a SIP entity that
 is "internal" to the intermediary before the same intermediary
 retargets the request to an external target.  A typical example would
 be a proxy that retargets a request first to a different user (i.e.,
 it maps to a different AOR) and then forwards it to a registered
 contact bound to the same AOR.  In this case, the intermediary MUST
 add an hi-entry for (each of) the internal target(s) per the
 procedures in Section 9.2.  The intermediary MAY include a Reason
 header field in the hi-entry with the hi-targeted-to-uri that has
 been retargeted.  Note that this is shown in the INVITE (F6) in the
 example entitled "Sequentially Forking (History-Info in Response)" in
 [CALLFLOWS].

8. Redirect Server Handling of History-Info Header Fields

 A redirect server MUST follow the procedures in Section 9.1 when it
 receives a SIP request.  A redirect server MUST follow the procedures
 in Section 9.4 when it sends a SIP response.  When generating the
 Contact header field in a 3xx response, the redirect server MUST add
 the appropriate "mp", "np", or "rc" header field parameter to each
 Contact header field as described in Section 10.4, if applicable.

Barnes, et al. Standards Track [Page 13] RFC 7044 History-Info February 2014

9. Handling of History-Info Header Fields in Requests and Responses

 This section describes the procedures for SIP entities for the
 handling of the History-Info header field in SIP requests and
 responses.

9.1. Receiving a Request

 When receiving a request, a SIP entity MUST keep a copy of the
 hi-entries from the incoming request.  This document describes this
 copy in terms of a cache containing the hi-entries associated with
 the request.  The hi-entries MUST be added to the cache in the order
 in which they were received in the request.
 If the Request-URI of the incoming request does not match the hi
 -targeted-to-uri in the last hi-entry (i.e., the previous SIP entity
 that sent the request did not include a History-Info header field),
 the SIP entity MUST add an hi-entry to the end of the cache, on
 behalf of the previous SIP entity.  This is done as follows, before
 proceeding to Section 9.2.
    The SIP entity MUST set the hi-targeted-to-uri to the value of the
    Request-URI in the incoming request.  If the Request-URI is a
    Tel-URI, it SHOULD be transformed into a SIP URI (per
    Section 19.1.6 of [RFC3261]) before being added as an
    hi-targeted-to-uri.
    If privacy is required, the SIP entity MUST follow the procedures
    of Section 10.1.
    The SIP entity MUST set the hi-index parameter as described in
    Section 10.3.
    The SIP entity MUST NOT include an "rc", "mp", or "np" header
    field parameter.

9.2. Sending a Request with History-Info

 When sending a request, a SIP entity MUST include all the hi-entries
 from the cache that was created per Section 9.1.  In addition, the
 SIP entity MUST add a new hi-entry to the outgoing request, but the
 SIP entity MUST NOT add the hi-entry to the cache at this time.  The
 hi-entries in the outgoing request's History-Info header field
 represent the preorder of the tree of hi-entries, that is, by the
 lexicographic ordering of the hi-indexes.  The new hi-entry is
 populated as follows:

Barnes, et al. Standards Track [Page 14] RFC 7044 History-Info February 2014

 hi-targeted-to-uri:  The hi-targeted-to-uri MUST be set to the value
    of the Request-URI of the current (outgoing) request.  If the
    Request-URI is a Tel-URI, it SHOULD be transformed into a SIP URI
    (per Section 19.1.6 of [RFC3261]) before being added as an
    hi-targeted-to-uri.
 privacy:  If privacy is required, the procedures of Section 10.1 MUST
    be followed.
 hi-index:  The SIP entity MUST include an hi-index for the hi-entry
    as described in Section 10.3.
 rc/mp/np:  The SIP entity MUST include an "rc", "mp", or "np" header
    field parameter in the hi-entry, if applicable, per the procedures
    in Section 10.4.

9.3. Receiving a Response with History-Info or Request Timeouts

 When a SIP entity receives a non-100 response or a request times out,
 the SIP entity performs the following steps:
 Step 1:  Add hi-entry to cache
    The SIP entity MUST add the hi-entry that was added to the request
    that received the non-100 response or timed out to the cache, if
    it was not already cached.  The hi-entry MUST be added to the
    cache in ascending order as indicated by the values in the
    hi-index parameters of the hi-entries (e.g., 1.2.1 comes after 1.2
    but before 1.2.2 or 1.3).
 Step 2:  Add Reason header field
    If the response is not a 100 or 2xx response, the SIP entity adds
    one or more Reason header fields to the hi-targeted-to-uri in the
    (newly) cached hi-entry reflecting the SIP response code in the
    non-100 or non-2xx response, per the procedures of Section 10.2.
 Step 3:  Add additional hi-entries
    The SIP entity MUST also add to the cache any hi-entries received
    in the response that are not already in the cache.  This situation
    can occur when the entity that generated the non-100 response
    retargeted the request before generating the response.  As per
    Step 1, the hi-entries MUST be added to the cache in ascending
    order as indicated by the values in the hi-index parameters of the
    hi-entries.

Barnes, et al. Standards Track [Page 15] RFC 7044 History-Info February 2014

 It is important to note that the cache (and the request or response)
 does not contain hi-entries for requests that have not yet received a
 non-100 response, so there can be gaps in indices (e.g., 1.2 and 1.4
 could be present but not 1.3).
 Note that in the case that a request has traversed one or more
 intermediaries that do not support RFC 4244 or this document, there
 can be duplicate indices (due to forking), which would be added to
 the appropriate position in the cache in the order in which they are
 received.

9.4. Sending History-Info in Responses

 When sending a response other than a 100, a SIP entity MUST include
 all the cached hi-entries in the response, subject to the privacy
 consideration in Section 10.1.2, and with the following exception: If
 the received request contained no hi-entries and there is no
 "histinfo" option tag in the Supported header field, the SIP entity
 MUST NOT include History-Info in the response.

10. Processing the History-Info Header Field

 The following subsections describe the procedures for processing the
 History-Info header field.  These procedures are applicable to SIP
 entities such as proxies/intermediaries, redirect servers, or user
 agents.

10.1. Privacy in the History-Info Header Field

 The privacy requirements for this document are described in
 Appendix A.2.  Section 10.1.1 describes the insertion of the Privacy
 header field (defined in [RFC3323]) to indicate the privacy to be
 applied to the History-Info header field entries.  Section 10.1.2
 describes how to apply privacy to a request or response that is being
 forwarded, based on the presence of the Privacy header field.

10.1.1. Indicating Privacy

 As with other SIP headers described in [RFC3323], the
 hi-targeted-to-uris in the History-Info header field can
 inadvertently reveal information about the initiator of the request.
 Thus, the UAC needs a mechanism to indicate that the
 hi-targeted-to-uris in the hi-entries need to be privacy protected.
 The Privacy header field is used by the UAC to indicate that privacy
 is to be applied to all the hi-entries in the request as follows:

Barnes, et al. Standards Track [Page 16] RFC 7044 History-Info February 2014

 o  If the UAC is including a Privacy header field with a priv-value
    of "header" in the request, then the UAC SHOULD NOT include a
    priv-value of "history" in the Privacy header field in the
    request.
 o  If the UAC is including any priv-values other than "header" in the
    Privacy header field, then the UAC MUST also include a priv-value
    of "history" in the Privacy header field in the request.
 o  If the UAC is not including any priv-values in the Privacy header
    field in the request, then the UAC MUST add a Privacy header
    field, with a priv-value of "history", to the request.  The UAC
    MUST NOT include a priv-value of "critical" in the Privacy header
    field in the request in this case.
 In addition, the History-Info header field can reveal general routing
 and diverting information that is within an intermediary and that the
 intermediary wants to privacy protect.  In this case, the
 intermediary MUST construct a Privacy header field with the single
 priv-value of "history" and include the Privacy header field in the
 hi-targeted-to-uri, for each new hi-entry created by the intermediary
 whose hi-targeted-to-uri it wishes to privacy protect.  Note that the
 priv-value in the Privacy header for the incoming request does not
 necessarily influence whether the intermediary includes a Privacy
 header field in the hi-entries.  For example, even if the Privacy
 header for the incoming request contained a priv-value of "none", the
 proxy can still set a priv-value of "history" in the Privacy header
 field included in the hi-targeted-to-uri.
 Finally, the UAS may not want to reveal the final reached target to
 the originator.  In this case, the UAS MUST include a Privacy header
 field with a priv-value of "history" in the hi-targeted-to-uri in the
 last hi-entry, in the response.  As noted above, the UAS of the
 request MUST NOT use any other priv-values in the Privacy header
 field included in the hi-entry.

10.1.2. Applying Privacy

 When a SIP message is forwarded to a domain for which the SIP
 intermediary is not responsible, a Privacy Service at the boundary of
 the domain applies the appropriate privacy based on the value of the
 Privacy header field in the message header or in the "headers"
 component of the hi-targeted-to-uri in the individual hi-entries.
 If there is a Privacy header field in the message header of a request
 or response, with a priv-value of "header" or "history", then all the
 hi-targeted-to-uris (in the hi-entries associated with the domain for
 which the SIP intermediary is responsible) are anonymized by the

Barnes, et al. Standards Track [Page 17] RFC 7044 History-Info February 2014

 Privacy Service.  The Privacy Service MUST change any
 hi-targeted-to-uris in these hi-entries that have not been anonymized
 (evidenced by their domain not being "anonymous.invalid") to
 anonymous URIs containing a domain of anonymous.invalid as
 recommended in Section 4.1.1.3 of [RFC3323].  As defined in
 Section 4.1.1.2 of [RFC3323], the recommendations of [RFC3261] for
 anonymizing the URI Username SHOULD be followed (i.e., "anonymous" in
 the user portion of the URI).  If there is a Privacy header field in
 the "headers" component of the hi-targeted-to-uri in the hi-entries,
 then the Privacy header field value MUST be removed from the
 hi-entry.  Once all the appropriate hi-entries have been anonymized,
 the Privacy Service MUST remove the priv-value of "history" from the
 Privacy header field in the message header of the request or
 response.  If there are no remaining priv-values in the Privacy
 header field, the Privacy Service MUST remove the Privacy header
 field from the request or response per [RFC3323].
 If there is not a Privacy header field in the message header of the
 request or response that is being forwarded, but there is a Privacy
 header field with a priv-value of "history" in the "headers"
 component in any of the hi-targeted-uris in the hi-entries associated
 with the domain for which a SIP intermediary is responsible, then the
 Privacy Service MUST update those hi-targeted-to-uris as described
 above.  Any other priv-values in the Privacy header field in the
 "headers" component of the hi-targeted-to-uris in the hi-entries MUST
 be ignored.  In any case, the Privacy Service MUST remove the Privacy
 header field from the "headers" component of the hi-targeted-to-uris
 in the hi-entries prior to forwarding.

10.2. Reason in the History-Info Header Field

 A Reason header field is added when the hi-entry is added to the
 cache based upon the receipt of a SIP response that is neither a 100
 nor a 2xx response, as described in Section 9.3.  The SIP entity MUST
 include a Reason header field, containing the SIP Response Code, in
 the "headers" component of the hi-targeted-to-uri in the last
 hi-entry added to the cache, unless the hi-targeted-to-uri is a
 Tel-URI.  In addition, if the response contains any Reason header
 fields (see [RFC3326]), then the SIP entity MUST also include the
 Reason header fields in the "headers" component of the
 hi-targeted-to-uri in the last hi-entry added to the cache.
 If a request has timed out (instead of being explicitly rejected),
 the SIP entity MUST update the cache as if the request received a SIP
 error response code of 408 "Request Timeout".

Barnes, et al. Standards Track [Page 18] RFC 7044 History-Info February 2014

 A request can receive multiple responses that are neither 100 nor 2xx
 responses and that carry or imply (for responses without Reason
 headers, and for timeouts) multiple, possibly duplicated,
 reason-values to be applied to an hi-targeted-to-uri.  In these
 situations, the SIP entity creating the History-Info header value
 would choose the appropriate Reason header field value.
 A SIP entity MAY also include a Reason header field (in the "headers"
 component of an hi-targeted-to-uri) that contains the URI of a
 request that was retargeted as a result of internal retargeting.
 If additional Reason header field parameters are defined in the
 future per [RFC3326], the use of these Reason header field parameters
 for the History-Info header field MUST follow the same rules as
 described above.

10.3. Indexing in the History-Info Header Field

 In order to maintain ordering and accurately reflect the retargeting
 of the request, the SIP entity MUST add an hi-index to each hi-entry.
 Per the syntax in Section 5, the hi-index consists of a series of
 nonnegative integers separated by dots (e.g., 1.1.2).  Each dot
 reflects a SIP forwarding hop.  The nonnegative integer following
 each dot reflects the order in which a request was retargeted at the
 hop.  The highest nonnegative integer at each hop reflects the number
 of entities to which the request has been retargeted at the specific
 hop (i.e., the number of branches) at the time that the request
 represented by this hi-entry was generated.  Thus, the indexing
 results in a logical tree representation for the history of the
 request and the hi-entries are given in the preorder of the tree.
 The first index in a series of History-Info entries MUST be set to 1.
 In the case that a SIP entity (intermediary or UAS) adds a first
 hi-entry on behalf of the previous hop, the hi-index MUST be set to
 1.  For each forward hop (i.e., each new level of indexing), the last
 integers of the hi-indexes of the new requests MUST be generated
 starting at 1 and incrementing by 1 for each additional request.
 The basic rules for adding the hi-index are summarized as follows:
 1.  Forwarding a request without changing the target: In the case of
     a request that is being forwarded without changing the target,
     the hi-index reflects the increasing length of the branch.  In
     this case, the SIP entity MUST read the value from the History-
     Info header field in the received request and MUST add another
     level of indexing by appending the dot delimiter followed by an
     initial value of 1 for the new level.  For example, if the

Barnes, et al. Standards Track [Page 19] RFC 7044 History-Info February 2014

     hi-index in the last History-Info header field in the received
     request is 1.1, a proxy would add an hi-entry with an hi-index of
     1.1.1 and forward the request.
 2.  Retargeting within a processing entity - first instance: For the
     first instance of retargeting within a processing entity, the SIP
     entity MUST calculate the hi-index as prescribed for basic
     forwarding.
 3.  Retargeting within a processing entity - subsequent instance: For
     each subsequent retargeting of a request by the same SIP entity,
     the SIP entity MUST calculate and add the hi-index for each new
     branch by incrementing the rightmost value from the hi-index in
     the last hi-entry.  Per the example above, the hi-index in the
     next request forwarded by this same SIP entity would be 1.1.2.
 4.  Retargeting based upon a response: In the case of retargeting due
     to a specific response (e.g., 302), the SIP entity MUST calculate
     the hi-index calculated per rule 3.  That is, the rightmost value
     of the hi-index MUST be incremented (i.e., a new branch is
     created).  For example, if the hi-index in the History-Info
     header field of the sent request is 1.2 and the response to the
     request is a 302, then the hi-index in the History-Info header
     field for the new hi-targeted-to-URI would be 1.3.
 5.  Forking requests: If the request forwarding is done in multiple
     forks (sequentially or in parallel), the SIP entity MUST set the
     hi-index for each hi-entry for each forked request per the rules
     above, with each new request having a unique index.  Each index
     MUST be sequentially assigned.  For example, if the index in the
     last History-Info header field in the received request is 1.1,
     this processing entity would initialize its index to 1.1.1 for
     the first fork, 1.1.2 for the second, and so forth.  (See
     Figure 1 for an example.)  Note that, in the case of parallel
     forking, only the hi-entry corresponding to the fork is included
     in the request because no response can yet have been received for
     any of the parallel forked requests.
 6.  Missing entry: If the request clearly has a gap in the hi-entry
     (i.e., the last hi-entry and Request-URI differ), the entity
     adding an hi-entry MUST add a single index with a value of "0"
     (i.e., the nonnegative integer zero) prior to adding the
     appropriate index for the action to be taken.  For example, if
     the index of the last hi-entry in the request received was 1.1.2
     and there was a missing hi-entry and the request was being
     forwarded to the next hop, the resulting index will be 1.1.2.0.1.
     In the case of requests that are forked by a proxy that does not
     support History-Info, it is possible for hi-entries generated by

Barnes, et al. Standards Track [Page 20] RFC 7044 History-Info February 2014

     different entities to have the same index, i.e., each entity
     supporting History-Info would receive a forked request with the
     same hi-index to which they would add the value of ".0" prior to
     adding the appropriate index.  Thus, in the previous example,
     each of the next-hop entities would generate an hi-index of
     1.1.2.0.1.

10.4. Mechanism for Target Determination in the History-Info Header

     Field
 This specification defines three header field parameters, "rc", "mp",
 and "np".  The header field parameters "rc" and "mp" indicate the
 mechanism by which a new target for a request is determined.  The
 header field "np" reflects that the target has not changed.  All
 parameters contain an index whose value is the hi-index of the
 hi-entry with an hi-targeted-to-uri that represents the Request-URI
 that was retargeted.
 The SIP entity MUST determine the specific parameter field to be
 included in the hi-target-param, in the History-Info header field, as
 the targets are added to the target set per the procedures in
 Section 16.5 of [RFC3261] or per Section 8.1.3.4 of [RFC3261] in the
 case of retargeting to a Contact URI received in a 3xx response.  In
 the latter case, the specific header field parameter in the Contact
 header field becomes the header field parameter that is used in the
 hi-entry when the request is retargeted.  If the Contact header field
 does not contain an "rc" or "mp" header field parameter, then the SIP
 entity MUST NOT include an "rc" or "mp" header field parameter in the
 hi-target-param in the hi-entry when the request is retargeted to a
 Contact URI received in a 3xx response.  This is because the redirect
 server is the only element with any knowledge on how the target was
 determined.  Note that the "np" header field parameter is not
 applicable in the case of redirection.
 Based on the following criteria, the SIP entity (intermediary or
 redirect server) determines the specific header field parameter
 ("rc", "mp", or "np") to be used.
 o  "rc": The Request-URI has changed while the target user associated
    with the original Request-URI prior to retargeting has been
    retained.
 o  "mp": The target was determined based on a mapping to a user other
    than the target user associated with the Request-URI being
    retargeted.
 o  "np": The target hasn't changed, and the associated Request-URI
    remained the same.

Barnes, et al. Standards Track [Page 21] RFC 7044 History-Info February 2014

 Note that there are two scenarios by which the "mp" header field
 parameter can be derived.
 o  The mapping was done by the receiving entity on its own authority,
    in which case the mp-value is the parent index of the hi-entry's
    index.
 o  The mapping was done due to receiving a 3xx response, in which
    case the mp-value is an earlier sibling or descendant of an
    earlier sibling of the hi-entry's index; the index is that of the
    downstream request that received the 3xx response.

11. Application Considerations

 History-Info provides a very flexible building block that can be used
 by intermediaries and UAs for a variety of services.  Prior to any
 application usage of the History-Info header field parameters, the
 SIP entity that processes the hi-entries MUST evaluate the hi-entries
 and determine if there are any gaps in the hi-entries.  The SIP
 entity MUST be prepared to process effectively messages whose
 hi-entries show evidence of "gaps", that is, situations that reveal
 that not all of the forks of the request have been recorded in the
 hi-entries.  Gaps are possible if the request is forwarded through
 intermediaries that do not support the History-Info header field and
 are reflected by the existence of hi-entries with a nonnegative
 integer of "0", e.g., "1.1.0.1".  Gaps are also possible in the case
 of parallel forking if there is an outstanding request at the time
 the SIP entity sends a message.  In addition, gaps may introduce the
 possibility of duplicate values for the hi-index in the case that a
 proxy that does not support History-Info forks a request.  If gaps
 are detected, the SIP entity MUST NOT treat this as an error but
 SHOULD indicate to any applications that there are gaps.  The
 interpretation of the information in the History-Info header field
 depends upon the specific application; an application might need to
 provide special handling in some cases where there are gaps.
 The following describes some categories of information that
 applications can use:
 1.  Complete history information, e.g., for debugging or other
     operational and management aspects, optimization of determining
     targets to avoid retargeting to the same URI, etc.  This
     information is relevant to proxies, UACs, and UASs.

Barnes, et al. Standards Track [Page 22] RFC 7044 History-Info February 2014

 2.  Hi-entry with the index that matches the value of the "rc" header
     field parameter in the last hi-entry with an "rc" header field
     parameter in the request received by a UAS, i.e., the last AOR
     that was retargeted to a contact based on an AOR-to-contact
     binding.
 3.  Hi-entry with the index that matches the value of the "mp" header
     field parameter in the last hi-entry with an "mp" header field
     parameter in the hi-target-param in the request received by a
     UAS, i.e., the last Request-URI that was mapped to reach the
     destination.
 4.  Hi-entry with the index that matches the value of the "rc" header
     field parameter in the first hi-entry with an "rc" header field
     parameter in the request received by a UAS.  Note that this would
     be the original AOR if all the entities involved support the
     History-Info header field and there is an absence of an "mp"
     header field parameter prior to the "rc" header field parameter
     in the hi-target-param in the History-Info header field.
     However, there is no guarantee that all entities will support
     History-Info; thus, the hi-entry that matches the value of the
     "rc" header field parameter of the first hi-entry with an "rc"
     header field parameter in the hi-target-param within the domain
     associated with the target URI at the destination is more likely
     to be useful.
 5.  Hi-entry with the index that matches the value of the "mp" header
     field parameter in the first hi-entry with an "mp" header field
     parameter in the request received by a UAS.  Note that this would
     be the original mapped URI if all entities supported the History-
     Info header field.  However, there is no guarantee that all
     entities will support History-Info; thus, the hi-entry that
     matches the value of the "mp" header field parameter of the first
     hi-entry with an "mp" header field parameter within the domain
     associated with the target URI at the destination is more likely
     to be useful.
 In many cases, applications are most interested in the information
 within one or more particular domains; thus, only a subset of the
 information is required.
 Some applications may use multiple types of information.  For
 example, an Automatic Call Distribution (ACD) / call center
 application that utilizes the hi-entry with an index that matches the
 value of the "mp" header field parameter in the first hi-entry with
 an "mp" header field parameter may also display other agents,
 reflected by hi-entries prior to hi-entries with an "rc" header field
 parameter, to whom the call was targeted prior to its arrival at the

Barnes, et al. Standards Track [Page 23] RFC 7044 History-Info February 2014

 current agent.  This could allow the agent the ability to decide how
 they might forward or reroute the call if necessary (avoiding agents
 that were not previously available for whatever reason, etc.).
 Since support for History-Info header field is optional, a service
 MUST define default behavior for requests and responses not
 containing History-Info header fields.  For example, an entity may
 receive an incomplete set of hi-entries or hi-entries that are not
 tagged appropriately with an hi-target-param in the case of entries
 added by entities that are only compliant to RFC 4244.  This may not
 impact some applications (e.g., debug); however, it could require
 some applications to make some default assumptions in this case.  For
 example, in an ACD scenario, the application could select the oldest
 hi-entry with the domain associated with the ACD system and display
 that as the original called party.  Depending upon how and where the
 request may have been retargeted, the complete list of agents to whom
 the call was targeted may not be available.

12. Application-Specific Usage

 The following are possible (non-normative) application-specific
 usages of History-Info.

12.1. PBX Voicemail

 A voicemail system (VMS) typically requires the original called party
 information to determine the appropriate mailbox so an appropriate
 greeting can be provided and the appropriate party notified of the
 message.
 The original target is determined by finding the first hi-entry
 tagged with "rc" and using the hi-entry referenced by the index of
 the "rc" header field parameter as the target for determining the
 appropriate mailbox.  This hi-entry is used to populate the "target"
 URI parameter as defined in [RFC4458].  The VMS can look at the last
 hi-entry and find the target of the mailbox by looking at the URI
 entry in the "target" URI parameter in the hi-entry.
 This example usage does not work properly in the presence of
 forwarding that takes place before the call reaches the company.  In
 that case, not the first hi-entry with an "rc" value, but the first
 hi-entry with an "rc" value following an "mp" entry needs to be
 picked.  Further detail for this example can be found in the call
 flow entitled "PBX Voicemail Example" in [CALLFLOWS].
 Note that in the case where there is no entry tagged with "rc", a VMS
 can follow the procedures, as defined in [RFC4458], for the
 "Interaction with Request History Information".

Barnes, et al. Standards Track [Page 24] RFC 7044 History-Info February 2014

12.2. Consumer Voicemail

 The voicemail system in this environment typically requires the last
 called party information to determine the appropriate mailbox so an
 appropriate greeting can be provided and the appropriate party
 notified of the message.
 The last target is determined by finding the hi-entry referenced by
 the index of the last hi-entry tagged with "rc" for determining the
 appropriate mailbox.  This hi-entry is used to populate the "target"
 URI parameter as defined in [RFC4458].  The VMS can look at the last
 hi-entry and find the target of the mailbox by looking for the
 "target" URI parameter in the hi-entry.  Further detail for this
 example can be found in the call flow entitled "Consumer Voicemail
 Example" in [CALLFLOWS].
 In the case where there is no entry tagged with "rc", a VMS can
 follow the procedures, as defined in [RFC4458], for the "Interaction
 with Request History Information".

13. Security Considerations

 The security requirements for this specification are specified in
 Appendix A.1.
 This document defines a header field for SIP.  The use of the
 Transport Layer Security (TLS) protocol [RFC5246] as a mechanism to
 ensure the overall confidentiality of the History-Info header fields
 (SEC-req-4) is strongly RECOMMENDED.  If TLS is NOT used, the
 intermediary MUST ensure that the messages are only sent within an
 environment that is secured by other means or that the messages don't
 leave the intermediary's domain.  This results in History-Info's
 having at least the same level of security as other headers in SIP
 that are inserted by intermediaries.  With TLS, History-Info header
 fields are no less, nor no more, secure than other SIP header fields,
 which generally have even more impact on the subsequent processing of
 SIP sessions than the History-Info header field.
 Note that while using the SIPS scheme (as per [RFC5630]) protects
 History-Info from tampering by arbitrary parties outside the SIP
 message path, all the intermediaries on the path are trusted
 implicitly.  A malicious intermediary could arbitrarily delete,
 rewrite, or modify History-Info.  This specification does not attempt
 to prevent or detect attacks by malicious intermediaries.

Barnes, et al. Standards Track [Page 25] RFC 7044 History-Info February 2014

 In terms of ensuring the privacy of hi-entries, the same security
 considerations as those described in [RFC3323] apply.  The Privacy
 Service that's defined in [RFC3323] MUST also support the new Privacy
 header field priv-value of "history" and anonymize hi-entries in the
 case of a priv-value of "header" as described in Section 10.1.2.

14. IANA Considerations

 IANA registrations have been implemented or updated as detailed in
 the following subsections.
 This document obsoletes [RFC4244] but uses the same SIP header field
 name, Privacy header field, and Option tag.  References to [RFC4244]
 in the IANA "Session Initiation Protocol (SIP) Parameters" registry
 (<http://www.iana.org/assignments/sip-parameters>) have been replaced
 with references to this document.

14.1. Registration of New SIP History-Info Header Field

 This document defines a SIP header field name, History-Info; and an
 option tag, histinfo.  The following updates have been made to
 <http://www.iana.org/assignments/sip-parameters>.
 The following row has been updated in the "Header Fields" sub-
 registry:
 Header Name             Compact Form               Reference
 -----------             ------------               ---------
 History-Info               none                    [RFC7044]
 The following has been updated in the "Option Tags" sub-registry:
 Name        Description                                 Reference
 ----        -----------                                 ---------
 histinfo    When used with the Supported header field,  [RFC7044]
             this option tag indicates the UAC supports
             the History Information to be captured for
             requests and returned in subsequent
             responses.  This tag is not used in a
             Proxy-Require or Require header field,
             since support of History-Info is optional.

Barnes, et al. Standards Track [Page 26] RFC 7044 History-Info February 2014

14.2. Registration of "history" for SIP Privacy Header Field

 This document defines a priv-value for the SIP Privacy header field:
 history.  The following updates have been made to the "SIP Privacy
 Header Field Values" sub-registry in <http://www.iana.org/assignments
 /sip-parameters> for the registration of the SIP Privacy header
 field:
 Privacy
 Type     Description             Registrant                 Reference
 ------   -----------             ----------                 ---------
 history  Privacy requested for   Mary Barnes                [RFC7044]
          History-Info header     mary.ietf.barnes@gmail.com
          field(s)

14.3. Registration of Header Field Parameters

 This specification defines the following new SIP header field
 parameters in the "Header Field Parameters and Parameter Values" sub-
 registry in <http:/www.iana.org/assignments/sip-parameters>.
  Header Field     Parameter Name   Predefined Values  Reference
 -------------     --------------   -----------------  ---------
  History-Info           mp                 No         [RFC7044]
  History-Info           rc                 No         [RFC7044]
  History-Info           np                 No         [RFC7044]
  Contact                mp                 No         [RFC7044]
  Contact                rc                 No         [RFC7044]
  Contact                np                 No         [RFC7044]

15. Acknowledgements

 Jonathan Rosenberg et al. produced the document that provided
 additional use cases precipitating the requirement for the new header
 parameters to capture the method by which a Request-URI is
 determined.  The authors would like to acknowledge the constructive
 feedback provided by Ian Elz, Paul Kyzivat, John Elwell, Hadriel
 Kaplan, Marianne Mohali, Brett Tate, and Dale Worley.  John Elwell
 also provided excellent suggestions in terms of document structure.
 Dan Romascanu performed the Gen-ART review.
 Mark Watson, Cullen Jennings, and Jon Peterson provided significant
 input into the initial work that resulted in the development of
 [RFC4244].  The authors would like to acknowledge the constructive
 feedback provided by Robert Sparks, Paul Kyzivat, Scott Orton, John
 Elwell, Nir Chen, Palash Jain, Brian Stucker, Norma Ng, Anthony
 Brown, Jayshree Bharatia, Jonathan Rosenberg, Eric Burger, Martin

Barnes, et al. Standards Track [Page 27] RFC 7044 History-Info February 2014

 Dolly, Roland Jesske, Takuya Sawada, Sebastien Prouvost, and
 Sebastien Garcin in the development of [RFC4244].
 The authors would like to acknowledge the significant input from
 Rohan Mahy on some of the normative aspects of the ABNF for
 [RFC4244], particularly regarding security and the index (the need
 for it as well as its format).

16. Changes from RFC 4244

 This RFC replaces [RFC4244].
 Deployment experience with [RFC4244] over the years has shown a
 number of issues, warranting an update:
 o  In order to make [RFC4244] work in "real life", one needs to make
    "assumptions" on how History-Info is used.  For example, numerous
    implementations filter out many entries and only leave specific
    entries corresponding, for example, to first and last redirection.
    Since vendors use different rules, this causes significant
    interoperability issues.
 o  [RFC4244] is overly permissive and evasive about recording
    entries, causing interoperability issues.
 o  The examples in the call flows had errors and were confusing
    because they often assume "loose routing".
 o  [RFC4244] has lots of repetitive and unclear text due to the
    combination of requirements with the solution.
 o  [RFC4244] gratuitously mandates the use of TLS on every hop.  No
    existing implementation enforces this rule, and instead, whether
    to use TLS is a general SIP issue, not an issue with [RFC4244]
    per se.
 o  [RFC4244] does not include clear procedures on how to deliver
    current target URI information to the UAS when the Request-URI is
    replaced with a contact.
 o  [RFC4244] does not allow for marking History-Info entries for easy
    processing by user agents.

Barnes, et al. Standards Track [Page 28] RFC 7044 History-Info February 2014

 The following summarizes the functional changes between this
 specification and [RFC4244]:
 1.  Added header field parameters to capture the specific method by
     which a target is determined to facilitate processing by users of
     the History-Info header field entries.  A specific header field
     parameter is captured for each of the target URIs as the target
     set is determined (per Section 16.5 of [RFC3261]).  The header
     field parameter is used in both the History-Info and the Contact
     header fields.
 2.  Added a way to indicate a gap in History-Info by adding a
     nonnegative integer of "0".
 3.  Rather than recommending that entries be removed in the case of
     certain values of the Privacy header field, the entries are
     anonymized.
 4.  Updated the security section to be equivalent to the security
     recommendations for other SIP header fields inserted by
     intermediaries.
 5.  Removed Appendix B ("Voicemail") since a separate call flow
     document is being published as a companion to this document.
 The first two changes are intended to facilitate application usage of
 the History-Info header field and eliminate the need to make
 assumptions based upon the order of the entries and ensure that the
 most complete set of information is available to the applications.
 In addition, editorial changes were done to both condense and clarify
 the text, moving the requirements to an appendix and removing the
 inline references to the requirements.  The examples were simplified
 and updated to reflect the protocol changes.  Several of the call
 flows in the appendix were removed and put into a separate document
 that includes additional use cases that require the new header field
 parameters.

16.1. Backwards Compatibility

 This specification is backwards compatible because [RFC4244] allows
 for the addition of new optional parameters.  This specification adds
 an optional SIP header field parameter to the History-Info and
 Contact header fields.  Entities that have not implemented this
 specification will ignore these parameters; however, per [RFC4244],
 an entity will not remove these parameters from an hi-entry.  While
 entities compliant to this document and [RFC4244] must be able to
 recognize gaps in the hi-entries, this document requires that an

Barnes, et al. Standards Track [Page 29] RFC 7044 History-Info February 2014

 index of "0" be used in this case.  In comparison, [RFC4244]
 recommended (but did not require) the use of "1".  However, since the
 ABNF in [RFC4244] defines the index as a DIGIT, "0" would be a valid
 value; thus, an [RFC4244] implementation should not have an issue if
 it receives hi-entries added by intermediaries compliant to this
 document.
 As for the behavior of the UACs, UASs, and intermediaries, the
 following additional normative changes have been made:
 UAC behavior
 1.  Inclusion of option tag by UAC has changed from SHOULD to MUST.
 2.  Inclusion of hi-target-entry along with hi-index has changed from
     MAY/RECOMMEND to MUST/MUST.
 3.  Behavior surrounding the addition of hi-target-entry based on a
     3xx response has changed from MAY/SHOULD to MUST.
 None of the behavior changes will cause any backward or forward
 compatibility issues.
 UAS behavior
 1.  Inclusion of hi-entry in response has changed from SHOULD to
     MUST.
 As the entity receiving response with hi-entry expected it with
 SHOULD, this change will not cause any backward compatibility issues.
 Proxy/redirect server behavior
 1.  Inclusion of the History-Info header field when forwarding the
     request has changed from SHOULD to MUST.
 2.  Association of Reason with timeout/internal reason has changed
     from MAY to MUST.
 3.  Inclusion of hi-index has changed from RECOMMENDED to MUST.
 4.  Inclusion of hi-entries in the response has changed from SHOULD
     to MUST.
 None of the above behavior changes impact backwards compatibility
 since they only strengthen normative behavior to improve
 interoperability.

Barnes, et al. Standards Track [Page 30] RFC 7044 History-Info February 2014

 In cases where an entity that is compliant to this document receives
 a request that contains hi-entries compliant only to RFC 4244 (i.e.,
 the hi-entries do not contain any of the new header field
 parameters), the entity MUST NOT add any of the new header field
 parameters to the hi-entries.  The hi-entries MUST be cached and
 forwarded as any other entries are, as specified in Section 9.1.  As
 with entities that are compliant to RFC 4244, applications must be
 able to function in cases of missing information, as specified in
 Section 11.

17. References

17.1. Normative References

 [RFC3261]   Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
             A., Peterson, J., Sparks, R., Handley, M., and E.
             Schooler, "SIP: Session Initiation Protocol", RFC 3261,
             June 2002.
 [RFC3326]   Schulzrinne, H., Oran, D., and G. Camarillo, "The Reason
             Header Field for the Session Initiation Protocol (SIP)",
             RFC 3326, December 2002.
 [RFC3323]   Peterson, J., "A Privacy Mechanism for the Session
             Initiation Protocol (SIP)", RFC 3323, November 2002.
 [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC5234]   Crocker, D. and P. Overell, "Augmented BNF for Syntax
             Specifications: ABNF", STD 68, RFC 5234, January 2008.
 [RFC5246]   Dierks, T. and E. Rescorla, "The Transport Layer Security
             (TLS) Protocol Version 1.2", RFC 5246, August 2008.
 [RFC4244]   Barnes, M., "An Extension to the Session Initiation
             Protocol (SIP) for Request History Information", RFC
             4244, November 2005.

17.2. Informative References

 [RFC5627]   Rosenberg, J., "Obtaining and Using Globally Routable
             User Agent URIs (GRUUs) in the Session Initiation
             Protocol (SIP)", RFC 5627, October 2009.
 [RFC5630]   Audet, F., "The Use of the SIPS URI Scheme in the Session
             Initiation Protocol (SIP)", RFC 5630, October 2009.

Barnes, et al. Standards Track [Page 31] RFC 7044 History-Info February 2014

 [RFC3087]   Campbell, B. and R. Sparks, "Control of Service Context
             using SIP Request-URI", RFC 3087, April 2001.
 [RFC4240]   Burger, E., Van Dyke, J., and A. Spitzer, "Basic Network
             Media Services with SIP", RFC 4240, December 2005.
 [RFC3966]   Schulzrinne, H., "The tel URI for Telephone Numbers", RFC
             3966, December 2004.
 [RFC4458]   Jennings, C., Audet, F., and J. Elwell, "Session
             Initiation Protocol (SIP) URIs for Applications such as
             Voicemail and Interactive Voice Response (IVR)", RFC
             4458, April 2006.
 [CALLFLOWS] Barnes, M., Audet, F., Schubert, S., Elburg, H., and C.
             Holmberg, "Session Initiation Protocol (SIP) History-Info
             Header Call Flow Examples", Work in Progress, November
             2013.

Barnes, et al. Standards Track [Page 32] RFC 7044 History-Info February 2014

Appendix A. Request History Requirements

 The following list constitutes a set of requirements for a "Request
 History" capability.
 1.  CAPABILITY-req: The "Request History" capability provides a
     capability to inform proxies and UAs involved in processing a
     request about the history/progress of that request.  Although
     this is inherently provided when the retarget is in response to a
     SIP redirect, it is deemed useful for non-redirect retargeting
     scenarios, as well.
 2.  GENERATION-req: "Request History" information is generated when
     the request is retargeted.
     A.  In some scenarios, it might be possible for more than one
         instance of retargeting to occur within the same proxy.  A
         proxy MUST also generate "Request History" information for
         the 'internal retargeting'.
     B.  An entity (UA or proxy) retargeting in response to a redirect
         or REFER MUST include any "Request History" information from
         the redirect/REFER in the new request.
 3.  ISSUER-req: "Request History" information can be generated by a
     UA or proxy.  It can be passed in both requests and responses.
 4.  CONTENT-req: The "Request History" information for each
     occurrence of retargeting shall include the following:
     A.  the new URI or address to which the request is in the process
         of being retargeted,
     B.  the URI or address from which the request was retargeted, and
         whether the retarget URI was an AOR,
     C.  the mechanism by which the new URI or address was determined,
     D.  the reason for the Request-URI or address modification, and
     E.  chronological ordering of the "Request History" information.
 5.  REQUEST-VALIDITY-req: "Request History" is applicable to requests
     not sent within an early or established dialog (e.g., INVITE,
     REGISTER, MESSAGE, and OPTIONS).

Barnes, et al. Standards Track [Page 33] RFC 7044 History-Info February 2014

 6.  BACKWARDS-req: "Request History" information may be passed from
     the generating entity backwards towards the UAC.  This is needed
     to enable services that inform the calling party about the dialog
     establishment attempts.
 7.  FORWARDS-req: "Request History" information may also be included
     by the generating entity in the request, if it is forwarded
     onwards.

A.1. Security Requirements

 The "Request History" information is being inserted by a network
 element retargeting a request, resulting in a slightly different
 problem than the basic SIP header problem, thus requiring specific
 consideration.  It is recognized that these security requirements can
 be generalized to a basic requirement of being able to secure
 information that is inserted by proxies.
 The potential security problems include the following:
 1.  A rogue application could insert a bogus Request History-Info
     entry by either adding an additional hi-entry as a result of
     retargeting or entering invalid information.
 2.  A rogue application could rearrange the "Request History"
     information to change the nature of the end application or to
     mislead the receiver of the information.
 3.  A rogue application could delete some or all of the "Request
     History" information.
 Thus, a security solution for "Request History" must meet the
 following requirements:
 1.  SEC-req-1: The entity receiving the "Request History" must be
     able to determine whether any of the previously added "Request
     History" content has been altered.
 2.  SEC-req-2: The ordering of the "Request History" information must
     be preserved at each instance of retargeting.
 3.  SEC-req-3: The entity receiving the information conveyed by the
     "Request History" must be able to authenticate the entity
     providing the request.
 4.  SEC-req-4: To ensure the confidentiality of the "Request History"
     information, only entities that process the request SHOULD have
     visibility to the information.

Barnes, et al. Standards Track [Page 34] RFC 7044 History-Info February 2014

 It should be noted that these security requirements apply to any
 entity making use of the "Request History" information.

A.2. Privacy Requirements

 Since the Request-URI that is captured could inadvertently reveal
 information about the originator, there are general privacy
 requirements that MUST be met:
 1.  PRIV-req-1: The entity retargeting the request must ensure that
     it maintains the network-provided privacy (as described in
     [RFC3323]) associated with the request as it is retargeted.
 2.  PRIV-req-2: The entity receiving the "Request History" must
     maintain the privacy associated with the information.  In
     addition, local policy at a proxy may identify privacy
     requirements associated with the Request-URI being captured in
     the "Request History" information.
 3.  PRIV-req-3: "Request History" information subject to privacy
     shall not be included in outgoing messages unless it is protected
     as described in [RFC3323].

Barnes, et al. Standards Track [Page 35] RFC 7044 History-Info February 2014

Authors' Addresses

 Mary Barnes
 Polycom
 TX
 US
 EMail: mary.ietf.barnes@gmail.com
 Francois Audet
 Skype
 EMail: francois.audet@skype.net
 Shida Schubert
 NTT
 EMail: shida@ntt-at.com
 Hans Erik van Elburg
 Detecon International Gmbh
 Sternengasse 14-16
 Cologne
 Germany
 EMail: ietf.hanserik@gmail.com
 Christer Holmberg
 Ericsson
 Hirsalantie 11, Jorvas
 Finland
 EMail: christer.holmberg@ericsson.com

Barnes, et al. Standards Track [Page 36]

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